The present invention relates generally to circuit breakers and more particularly to a circuit breaker employing a thermal-magnetic trip unit having an over centering mechanism for unlatching the circuit breaker operating mechanism and a trip flag system that discriminates between a short circuit trip and an overcurrent trip.
Circuit breakers typically provide protection against persistent overcurrent and against very high currents produced by short circuits. This type of protection is provided in many circuit breakers by a thermal-magnetic trip unit having a thermal trip portion, which trips the circuit breaker on persistent overcurrent conditions, and a magnetic trip portion, which trips the circuit breaker on short-circuit conditions.
In order to trip the circuit breaker, the thermal magnetic trip unit must activate an operating mechanism. Once activated, the operating mechanism separates a pair of main contacts to stop the flow of current in the protected circuit. Conventional trip units act directly upon the operating mechanism to activate the operating mechanism. In current thermal-magnetic trip unit designs, the thermal trip portion includes a bimetallic strip (bimetal), which bends at a predetermined temperature. The magnetic trip portion includes an anvil disposed about a current carrying strap and a lever disposed near the anvil, which is drawn towards the anvil when high, short-circuit currents pass through the current carrying strap. The force created by the bimetal or lever, and the distance that they travel, may be insufficient to directly trip the operating mechanism. A conventional way to solve this problem is to use a latch system as a supplemental source of energy. However, the drawback of a latch system is the use of latching surfaces, which degenerate over repeated use.
Further, a circuit breaker having a thermal-magnetic trip unit can be tripped by three events, namely: overcurrent, short circuit and ground fault. It is important to know the cause due to which a breaker has tripped. Distinguishing the reasons for tripping allows the user to determine if the breaker can be reset immediately, as in the case of an overcurrent, or only after careful inspection of the circuitry, as in the case of a short circuit or ground fault.
Circuit breaker trip mechanisms of the prior art have solved this problem by the use of flags, which are visible through windows disposed in the case of the circuit breaker. In such trip mechanisms, a flag appears in one window upon the occurrence of an overcurrent condition, while another flag appears in another window upon the occurrence of a short-circuit condition. This solution works well for trip units having an inactive bimetal. That is, for trip units where the bimetal does not carry electrical current, but is attached to a current-carrying strap. However, this solution can provide indeterminate indications when it is used with a trip unit having an active bimetal. That is, when it is used with a trip unit where the bimetal carries electrical current. When such an active bimetal is used, it is possible during a short circuit event that, in addition to the magnetic trip portion, the bimetal also moves to expose the overcurrent flag, thereby leading to both the short-circuit and overcurrent flags being shown thus providing an indeterminate indication to the user.